This invention relates to a semiconductor laser driver circuit and, more particularly, to a semiconductor laser driver circuit for suppressing jitter (pattern jitter) produced in dependence upon the signal pattern of input data, as a result of which there is no delay in light emission even when a semiconductor laser having a large threshold current is used.
An optical transmission apparatus that relies upon a semiconductor laser makes it possible to realize a transmission rate of several gigabits per second over a distance of tens of kilometers in a trunk-line system. Further, attempts have been made to perform transmission at a rate of a hundred megabits per second or more by multiplexing subscriber information in subscriber system communications.
The receiving section of an uplink optical transmission apparatus in a subscriber system collects signals from a number of subscribers. Consequently, if modulation of a semiconductor laser is carried out in the transmitting section on the subscriber side under conditions in which a bias current is being passed through the semiconductor laser at all times, the difference between the "1" and "0" levels of a received signal in the receiving section of the optical transmission apparatus will be too small. That is, the distinction ratio becomes so small as to degrade the reception characteristic.
FIG. 24 is a diagram useful in describing modulation of a semiconductor laser. Shown in FIG. 24 are a current vs. optical-power characteristic 101 of a semiconductor laser, in which Id represents diode current, P.sub.0 the output power of the semiconductor laser, Ith the threshold current and Ib the bias current. If a pulsed current of the kind indicated by the solid line 102 is passed through a semiconductor laser in accordance with the "1", "0" logic levels of data, the semiconductor laser will output signal light that is intensity-modulated in the manner indicated at 103. Thus, if modulation of the semiconductor laser is performed in the presence of the bias current, the difference between the "1" and "0" levels diminishes, the distinction ratio decreases and the problem set forth above arises.
Accordingly, zero-biased modulation indicated by the phantom lines in FIG. 24 generally is used in the transmitting section on the subscriber side. With zero-biased modulation, however, the light emission is delayed when use is made of a semiconductor laser having a large threshold current Ith. Even if a semiconductor laser having a small threshold current Ith is employed, the light emission is still delayed when the transmission speed rises.
FIG. 25 illustrates graphically the pulse-modulated light output of a semiconductor laser. The waveform indicated by the dashed line is a modulated waveform in a case where a DC bias current in the vicinity of or greater than the threshold current has been applied. The solid line indicates a modulated waveform in a case where no DC bias current has been applied (i.e., at the time of zero bias). It will be appreciated from FIG. 25 that the light emission at the rising edge of the curve is delayed in so-called zero-bias modulation. The emission delay may be considered to be the time required for the filling of carriers commensurate with the threshold current.
In the case of an actual signal, it is necessary to regard strings of pulses in a pulse train as being random, and it is known that the emission delay varies depending upon the pattern of pulses in the pulse train. This is caused by a fluctuation in the level of residual carriers that remain in the semiconductor laser, the fluctuation depending upon whether the immediately preceding train of pulses is composed of "0"s or "1" s. A case in which there are many of such residual carriers approximates a case where a DC bias is applied, resulting in a shorter emission delay time. Conversely, a case in which there are few residual carriers approximates pure zero-bias modulation, resulting in a longer emission delay time. This phenomenon is referred to as the "pattern effect" and is known as pattern jitter in zero-bias modulation. Pattern jitter is a major problem in that it causes error in terms of transmission signal identification and detracts from the reliability of an optical transmission apparatus.
In order to prevent such pattern jitter, methods adopted heretofore include a method of adding a waveform, which is obtained by integrating the pulse itself, onto the rising edge of the pulsed modulating signal to hasten the rise time of the modulating signal and broaden the pulse width, thereby compensating for the emission delay time of the waveform after modulation; a method of applying undershoot to the falling edge of the pulsed modulating signal to forcibly extract residual carriers; and a method of broadening pulse width of the modulating signal by superposing waveforms before and after delay using a delay element having the same delay as the emission delay, thereby compensating for the emission delay. These methods, however, do not make it possible to suppress pattern jitter that is dependent upon a fluctuation in emission delay.